In semiconductor technology there are various applications in which it is necessary to deposit patterned metal layers. Metal layers may be required, for example, in predetermined areas on the surface of the semiconductor structure. One example in which a patterned metal layer is deposited is the formation of a metal electrode for a sensor. Such a sensor is known from I. Lundstrom, S. Shivaraman, C. Svensson, and L. Lundkvist, "Hydrogen sensitive MOS field-effect transistor", Appl. Phys. Lett. 26, 55-57. The metal electrodes are commonly formed from noble metals, such as palladium, gold, or platinum.
In practice, the formation of such a patterned metal layer presents particular problems if it is to be deposited on an MOS structure, since the formation of such a layer cannot be combined with the conventional MOS process. After the MOS process, additional process steps would be necessary to deposit the patterned metal layer, particularly photolithographic steps, etching steps, and resist-stripping steps.
If such a metal electrode is deposited on an MOS structure, the process steps required will result in the structure being contaminated by the metal. This will impair the properties of silicon oxide and, in the case of MOS transistors, particularly the properties of the gate oxide, and of the underlying channel. Furthermore, the apparatus used to carry out the process steps in the semiconductor plant will be contaminated, which is not permissible in MOS fabrication. If a metal electrode of a base metal, such as aluminum or titanium, is deposited, these metals will change their properties during fabrication and on contact with other materials. As a result, the characteristics of the sensor will change as well. If platinum is used for the metal electrode, a bonding agent is required, for which titanium is used. In that case, the unfavorable properties described above will occur. Furthermore, the additional steps involved in depositing the metal electrode will make the process complex and costly.
Therefore, it is an object of the invention is to provide an economical process for the fabrication of a semiconductor device with a patterned metal layer.